Many polymers and particularly impact-modifying agents for polymers are formed by homo- or copolymerizing suitable monomers in a liquid medium by suspension, emulsion or precipitation polymerization. The polymer is formed therein as a usually aqueous dispersion of solids (latex) whence the polymer has to be removed if the latex is not used as such.
The polymer is typically removed from the dispersion by coagulation followed by a solid-liquid separation. Various methods are known for this, as described in Houben-Weyl, Methoden der organischen Chemie. Methods of removal can be subdivided into three main groups:
a) removing the aqueous phase in gaseous form
b) removing the aqueous phase in liquid form by admixture of auxiliaries
c) removing the aqueous phase in liquid form without admixture of auxiliaries
With a) and b) there is an in-principle problem in that the quality of the plastic can be impaired, since auxiliaries for the polymerization (e.g. emulsifiers) and/or the auxiliaries for the removal process can remain in the polymer to some extent and thus may cause adverse secondary reactions, for example yellowing or haze, in the later plastics molding.
DE 19718597 C1 describes a two-step process for dewatering synthetic resin dispersions comprising a) coagulating the two-phase liquid mixture in a first extruder and b) dewatering the coagulate in a second extruder.
U.S. Pat. No. 4,591,632 describes the process of freeze coagulation on cold surfaces.
Freeze Coagulation of ABS Latex, Adler et al., Ind. Eng. Chem. Res. 1997, 36, 2156, describes the freeze coagulation of ABS latices by contact with a cold surface and with liquid CO2.
DE 32 30 128 A1, “Freeze Coagulation of Polymer dispersions”, Bayer Lev., 1982, describes the process of freeze coagulation by application of shear stress to a moving layer of product.
EP 0467288 B1 describes the discontinuous freeze coagulation of acrylic polymer latices whence plastics moldings having good optical properties and good haze characteristics on exposure to water are obtainable. It is reported therein that freezing rates of 4 cm/hour or less are preferable, since these conditions make good solid-liquid separation possible and thus make it possible to co-remove auxiliaries for the polymerization (e.g. emulsifiers).